1. Field of the Invention
This invention relates to a high-temperature strength member used in a high-temperature exposed portion of a gas turbine, a jet engine or the like, and more particularly to a high-temperature strength member constructed by forming a film for preventing the lowering of high-temperature strength due to plastic work strain on a surface of a dynamic-static blade substrate made of Ni-based single crystal alloy or Ni-based unidirectional solidified alloy.
2. Description of Related Art
Recently, studies on the gas turbine are made for raising a working gas temperature in order to improve a heat efficiency, and hence a temperature of an inlet port in the turbine becomes higher than 1500xc2x0 C. at the present time, and it is demanded to develop a further temperature raising technique.
Such a temperature raising technique of the gas turbine is largely dependent upon the advance of a material for a turbine blade member directly exposed to a high-temperature combustion gas (including development of coatings for resistance to high-temperature oxidation and heat shielding) and the development of a technique of cooling the blade, and is also an important studying matter up to the present.
Particularly, the turbine blade is a central matter on the study of the blade member because it is subjected to creep through centrifugal force under an operating environment, thermal fatigue through start and stop of the turbine, high cycle fatigue through mechanical vibrations, and further corrosion action through impurities included in combustion gas such as sea salt particles, sulfur, vanadium and the like.
The outline of the state studying and developing the conventional turbine blade member is summarized as follows.
(1) Strengthening of alloys through precipitation-dispersion of a great amount of an intermetallic compound [Ni3(Al, Ti)] called as xcex3xe2x80x2-phase;
(2) Development of alloying process considering solid-solution strengthening of matrix phase xcex3 and xcex3xe2x80x2-phase and atomic arrangement of crystal boundary bade on a delicate balance of compositions in both phases, and development of alloys utilizing the success;
(3) Establishment of production method of high quality alloys by removing influence of slight impurities, gas and the like through adoption of vacuum melting technique;
(4) Development of high performance blade member through conversion from cast shaping to precision casting technique (enlargement of freedom degree in the field of cooling mechanism);
(5) Production of columnar crystal blade member from equiaxed crystal through development of unidirectional solidifying process of alloy;
(6) Development of a single crystal blade member solving the deterioration of material strength resulted from crystal grain boundary of polycrystalline alloy;
(7) A chemical composition of the single crystal blade member comprises Ni: 55-70 mass % as a main component and further contains other elements of Cr: 2-15 mass %, Co: 3-13 mass %, Mo: 0.4-8 mass %, W: 4.5-8 mass %, Ta: 2-12 mass %, Re: 3-6 mass %, Al: 3.4-6 mass %, Ti: 0.2-4.7 mass %, Hf: 0.04-0.2 mass %, C: 0.06-0.15 mass %, B: 0.001-0.02 mass %, Zr: 0.01-0.1 mass %, Nb: 0.1-1.5 mass % and the like. In these alloys, Cr and Al contents effective for resistance to high-temperature oxidation is relatively small, so that an excellent high-temperature strength is first developed by applying a surface-treated coating having a resistance to high-temperature oxidation and resistance to high-temperature corrosion (hereinafter referred to as resistance to high-temperature environment).
(8) To a high-temperature exposed member such as a gas turbine, a jet engine or the like is further applied an alloy coating having an excellent resistance to high-temperature oxidation called as xe2x80x9cMCrAlX alloyxe2x80x9d. In this case, M is Ni, Co or Fe alone, or an alloy consisting of these plural elements and X is an element of Y, Hf, Sc, Ce, La, Th, B or the like.
Even in such MCrAlX alloy, there are many proposals having various chemical compositions in accordance with use purposes. Prior techniques as to these alloys are mentioned as follows:
JP-A-58-37145, JP-A-58-37146, JP-A-59-6352, JP-A-59-89745, JP-A-50-29436, JP-A-51-30530, JP-A-50-158531, JP-A-51-10131, JP-A-52-33842, JP-A-55-115941, JP-A-53-112234, JP-A-52-66836, JP-A-52-88226, JP-A-53-33931, JP-A-58-141355, JP-A-56-108850, JP-A-54-16325, JP-A-57-155338, JP-A-52-3522, JP-A-54-66342, JP-A-59-118847, JP-A-56-62956, JP-A-51-33717, JP-A-54-65718, JP-A-56-93847, JP-A-51-94413, JP-A-56-119766, JP-A-55-161041, JP-A-55-113871, JP-A-53-85829, JP-A-57-185955, JP-A-52-117826, JP-A-60-141842, JP-A-57-177952, JP-A-59-1654.
These alloys have been mainly developed as a coating for polycrystalline alloy blade member having a resistance to high-temperature environment, but they are effective for a single crystal alloy or a unidirectional solidified alloy and are widely adopted.
On the other hand, Ni-based single crystal alloy and Ni-based unidirectional solidified alloy (hereinafter abbreviated as single crystal alloy, unidirectional solidified alloy simply) have a characteristic that when it is heated to a higher temperature at a state of subjecting to fatigue or thermal fatigue damage by plastic working or impact or further under an actual operating environment as a turbine blade, a portion of residual strain based on working or impact is modified to form a modified layer (see FIG. 4(a)-FIG. 4(d)). This modified layer portion is considered to be an aggregate of fine crystals, which can not be determined by the observation with an optical microscope, or a preparatory state thereof, but is confirmed by the inventors"" experiment that it is very brittle and simply forms many small cracks under a slight stress as a start point of breakage (see FIG. 5(a)-FIG. 5(b)).
Heretofore, there has been studied no technique that the decrease of the high-temperature strength resulted from the modified layer appearing on the surface of the substrate is prevented by surface coating. It is well-known that the aforementioned MCrAlX alloy coating is only applied to the improvement of the resistance to high-temperature environment targeting corrosion damage resulted from the high-temperature combustion gas.
The invention is to solve the following subject matters inherent to the blade member made of the single crystal alloy or unidirectional solidified alloy by the formation of sprayed coating or vapor deposition coating.
(i) The blade member of the single crystal alloy or unidirectional solidified alloy has a characteristic that when it is subjected to slight machining strain, roughening through blast treatment and the like at the production step, in the operation as the turbine blade, at the formation step of the protection coating and the like and then heated to a higher temperature, many fine crystal are produced on an affected portion to form a modified layer. This modified layer is brittle and produces many fine cracks under an application of small stress, which considerably deteriorate the high-temperature strength.
(ii) When only the conventional MCrAlX alloy sprayed coating is formed on the surface of the blade member made of the single crystal alloy or unidirectional solidified alloy at a state of subjecting to strain or machining, there can not be prevented the deterioration of the high-temperature strength accompanied with the formation of the above modified layer.
(iii) As a result, even in the dynamic and static blade members made of the single crystal alloy or unidirectional solidified alloy having an excellent high-temperature strength in a material technology, there is a state that the significance can not be sufficiently developed in the present technique.
The invention is to prevent the above problem inherent to the single crystal alloy or unidirectional solidified alloy as a high-temperature strength member, i.e. the deterioration of the high-temperature strength resulted from the breakage of crystal control (recrystallization in a broad meaning) induced by plastic working through a surface coating, and is developed based on the following technical ideas:
(a) When a coating of B-containing alloy containing B: 0.1-5 mass % is formed on a surface of a single crystal alloy or a unidirectional solidified alloy, as the alloy is heated, B diffuses and penetrates from the coating into an alloy substrate to enhance an interbonding force at recrystallized grain boundary, whereby the deterioration of the high-temperature strength in the alloy is prevented;
(b) After an undercoat comprised of the above B-containing alloy coating is formed on the surface of the single crystal alloy or unidirectional solidified alloy, an overcoat made of an alloy obtained by adding at least one element selected from Y, Hf, Ta, Cs, Ce, La, Th, W, Si, Pt and Mn to an alloy containing at least two of Co, Ni, Cr and Al (hereinafter referred at as MCrAlX alloy simply) is laminated on the undercoat, whereby the deterioration of the high-temperature strength resulted from the recrystallization phenomenon of the alloy is prevented and the resistance to high-temperature environment is improved with the MCrAlX alloy coating (overcoat);
(c) The surface of the overcoat made of the MCrAlX alloy is subjected to Al diffusion penetration treatment by CVD process or powder process to further improve the resistance to high-temperature environment in the overcoat;
(d) On the undercoat formed on the surface of the substrate is formed the overcoat made of the MCrAlX alloy, and a topcoat made of ZrO2 based ceramic containing at least one oxide selected from Y2O3, CaO, MgO, CeO2, Yb2O3, Sc2O3 and the like, whereby the high-temperature strength of the substrate is maintained and further the resistance to high-temperature environment is provided;
(e) On the surface of the substrate is formed a mixed coating of B-containing alloy and MCrAlX alloy, whereby the deterioration of the high-temperature strength due to the recrystallization of the single crystal alloy or the unidirectional solidified alloy is prevented and the resistance to high-temperature environment is improved.
In the invention, as the B-containing alloy formed on the surface of the substrate as an undercoat, it is preferable to use ones comprising 0.1-5.0 mass % of B and the remainder being two or more elements selected from Cr, Ni, Go, Mo, Al, Ta, W, Re, Zr, Hf, Y, Si, Pt, Fe and Ce and further being added with 0.01-1.5 mass % of C.
In the invention, the formation of the B-containing alloy coating (undercoat) formed on the surface of the substrate is carried out by a spraying process or a vapor deposition process, wherein it is preferable that a thickness of the coating is 2-150 xcexcm and a content of oxygen in the coating is less than 1.5 mass % and the B-containing alloy is formed on the substrate surface at a closed state.